43 Digestion and Absorption Flashcards
Name 3 structural modifications for Intestinal Digestion & Absorption
- Folding of intestinal surface
- Presence of villi
- Presence of microvilli
What are the functions of villus cells? What is the disease if there is loss of villi?
Digestion, Absorption, Secretion;
Celiac disease
What are the 2 functions of crypt cells?
- Undergo mitosis to replace worn out villus cells (high turnover rate)
- Active secretion
(over-secretion via CFTR channels might cause secretory diarrhea
What are luminal enzymes and membrane-bound enzymes?
Luminal enzymes are from salivary glands, stomach and pancreas.
Membrane-bound enzymes are hydrolytic enzymes inserted in brush border.
Which of the followings is/are not membrane-bound enzyme?
- Enterokinase
- Amylase
- Pepsin
- Disaccharidases
- Phospholipase A2
- Peptidases
Amylase
Pepsin
Phospholipase A2
are all luminal enzymes
How is auto-digestion in pancreas prevented?
Hint: Trypsinogen > Tyrpsin
Duodenal bound (membrane-bound) enzyme enterokinase cleaves trypsinogen to trypsin. All precursors (trypsinogen) are inactive in the pancreas.
Trypsin is autocatalytic and will undergo positive feedback, causes the rest trypsinogen to be mature
What is the disease arises from the premature of trypsinogen in the pancreas causing auto digestion in the acinar cells ?
Acute pancreatitis
How is auto-digestion prevented by the chief cells?
Chief cells secrete pepsinogen in stomach wall into the gastric lumen, which becomes fully active form in the mucosal surface & begins initial protein digestion.
Pepsinogen exposes its active site only when under pH stimulation
Also autocatalytic and causes the rest of the pepsinogen to become pepsin
List all examples of Monosaccharides, Oligosaccharides and Polysaccharides
Monosaccharides: glucose, galactose, fructose
Oligosaccharides: maltose ,lactose, sucrose
Polysaccharides: Glycogen, starch, cellulose*
*cellulose: indigestible due to the presence of beta1-4 peptide bond, BUT acts as dietary fibre or roughage that stimulates intestinal motility and thus prevents constipation
How is glucose absorbed?
By secondary active transport which is Na+ dependent; Requires SGLT1 (Sodium dependent glucose transporter)
How is galactose absorbed?
By secondary active transport which is Na+ dependent; Requires SGLT1 (Sodium dependent glucose transporter)
Same as glucose
How is fructose absorbed?
Facilitated diffusion;
Separate transport: GLUT5
Therefore, SGLT 1 defect causes glucose and galactose malabsorption, but normal fructose absorption is assured as the transport mechanism is different.
Alpha-amylases can cut:
A. a-1,4 glycosidic linkage
B. a-1,6-linkage
C. Both linkages
A
What are the differences between amylose and amylopectin?
In terms of branching, Amylose: unbranched (straight) chain of D-glucose (only contains a-1,4-linkage) Amylase: highly branched (with a-1,4 and a-1,6-linkages)
In terms of enzymes needed for degradation,
Amylose requires a-amylase;
Amylase requires membrane bound enzymes: a-dextrin
a-dextrin includes:
- A-dextrinase break a-1,6-linkages
- Glucoamylase break a-1,4-linkages
Which kind of Na+ dependent carrier transporter is predominantly expressed in SI?
SGLT 1
Glucose and galactose: competitive absorption
Which of the following statements are true:
A. Glucose absorption facilitates Na + transport
B. Na+ transport facilitates glucose absorption
Both A and B, by creating a concentration gradient
Which transporter created the Na+ gradient?
Na+/K+ ATPase
by pumping 3Na+ out and 2K+ in
After glucose and galactose entered the enterocyte, it enters the blood via \_\_\_\_\_\_ by \_\_\_\_\_\_\_\_. A. GLUT5; facilitated diffusion B. GLUT 2; facilitated diffusion C. GLUT 5; active transport D. GLUT 2; active transport
B
What does oral rehydration therapy contain? What is it for? How does it achieve this goal?
Glucose containing saline solution,
1 glucose/AA > Na+ and Cl- > 3 osmotically active molecules
For secretory/ watery diarrhea
Because glucose facilitates Na+ absorption, no more diarrhea
Partial complete digestion is achieved by ______ enzyme while complete digestion is achieved by ________ enzyme.
Why can the above enzyme achieve complete digestion?
Luminal enzyme (esp secreted by the pancreas);
Membrane-bound enzyme
Overlapping specificity.
(e.g. sucrase 25% specificity to digest maltose, 100 % specificity to digest sucrose)
Lactase is 100% specific to lactose, thus lactase deficiency will lead to malabsorption of lactose, causing lactose intolerance > diarrhea
What isoform of amino acid is absorbable by humans?
L-isoform
What type of amino acid do Phe, Tyr, Glr belong to?
Neutral
Basic: Lys, Arg (with additional NH3+)
Acidic: Glu, Asp (with additional COO-)
Other than neutral, what is the last type of aa?
List 2 examples of amino acid of this group.
Imino;
Proline, beta and y amino acids
e.g. GABA
What are the 2 sources of protein for digestion?
- Dietary (Exogenous) protein
2. Enzymes and mucus (Endogenous) from GI secretion
What are the 2 types of peptidases? What are they?
Endopeptidases
- luminal enzymes from stomach and pancreas
- Hydrolyses interior peptide bonds
Exopeptidases
- Carboxypeptidases from pancreas
- Aminopeptidases from membrane bound enterocytes
- Hydrolyses external peptide bonds
What are the 3 absorbable forms of proteins? What transporter is used respectively?
Free amino acids:
Na+ dependent amino acid transporter (40%)
Dipeptides:
Na+ dependent peptide transporter
Tripeptides:
Na+ dependent peptide transporter
Di+ Tri: 60%
How are proteins absorbed in newborns?
By pinocytosis, immunologically significant
What are the 4 categories of free amino acid carrier?
- Neutral (NBB, Neutral brush border)
- Basic (Cationic NH+)
- Acidic (Anionic COO-)
- Imino
- Neutral (NBB, Neutral brush border):
Aromatic (Phe, Tyr, Trp)
Aliphatic (Ala, Gly, Leu, Ile, Val, Ser, Thr, Cys, Met) - Basic (Cationic NH+):
Lys, Arg - Acidic (Anionic COO-):
Glu, Asp - Imino:
pro
60%: Small peptides entering the _______ membrane via Na+ dependent peptide transporters.
40%: Free amino acids entering the ________ membrane via Na+ dependent amino acid transporters.
In the _________, only simple amino acids can be absorbed, and thus oligopeptides must become amino acids to be absorbed.
Brush-border membrane/ Apical membrane;
Brush-border membrane/ Apical membrane;
Basolateral membrane (BLM)
3 genetic orders of amino acid transport due to defects in BBM or BLM.
- Hartnup’s disease
- Cystinuria
- Lysinuric protein intolerance
What causes Hartnup’s disease? How it can be treated?
The lack of free tryptophan.
Still can be absorbed as oligopeptides and transformed to free tryptophan as an aid, but still insufficient.
Treatment: more proteins intake/ more free tryptophan intake
What causes Cystinuria? How it can be treated?
The defect in amino acid transporters and (cysteine transporters).
Cysteine can still be absorbed in the form of oligopeptides as an aid
Treatment: more proteins intake/ more free cysteine intake
What causes Lysinuric protein intolerance? How it can be treated?
The defect in amino acid transporter (lysine transporters).
Can only be treated with lysine infusion.
